Method for electrostatically holding particles along a mold surface

ABSTRACT

Particles of plastic material are attracted to a mold surface by impressing an electrostatic charge on the particles subsequent to their deposit, minimizing disturbance of the deposition stream and sticking of powder in the deposition apparatus. A roll deposition unit and corona bar are employed in association with a rotating mold to form a cylindrical member from powdered plastic material. The unit deposits powder in a strip zone lengthwise of the mold. The corona bar is positioned externally of the roll case, parallel to and coextensive with the roll and closest to the cylindrical mold wall in a plane outside the stream of powder flowing from the roll to the mold wall. When the bar is charged and the mold is grounded, the resultant discharge is largely concentrated outside the stream, minimizing electrostatic effect on particles in the stream.

United States Patent Timko June 27, 1972 [54] METHOD FORELECTROSTATICALLY HOLDING PARTICLES ALONG A MOLD SU RFAC E [72]Inventor: Charles A. Timko, Westmont, ill.

[73] Assignee: Continental Can Company, Inc., New

York, NY.

[22] Filed: May 9, 1969 [2 i 1 Appl. No.: 826,781

(52] 11.5. CL ..264/24, 264/126, 264/310 [5| Int. Cl. .3291 5/02 [58]Field of Search ..264/24, 26, 126, 310

[56] References Cited UNITED STATES PATENTS 3,301,925 1/1967 Engel..264/37 3,491,170 1/1970 Roe, Jr. ..264/26 Primary Examiner-Robert F.White Assistant Examiner-J. R, Hall Att0rney-George E. Szekely, JosephE. Kerwin and William A. Dittmann [57] ABSTRACT Particles of plasticmaterial are attracted to a mold surface by impressing an electrostaticcharge on the particles subsequent to their deposit, minimizingdisturbance of the deposition stream and sticking of powder in thedeposition apparatus. A roll deposition unit and corona bar are employedin association with a rotating mold to form a cylindrical member frompowdered plastic material. The unit deposits powder in a strip zonelengthwise of the mold. The corona bar is positioned externally of theroll case, parallel to and coextensive with the roll and closest to thecylindrical mold wall in a plane outside the stream of powder flowingfrom the roll to the mold wall. When the bar is charged and the mold isgrounded, the resultant discharge is largely concentrated outside thestream, minimizing electrostatic effect on particles in the stream.

5 Claims, 4 Drawing Figures PATENTEDJUN27 m2 SHEET 2 OF 2 INVENTORCHARLES A. TIMKO BY 2W 3 TT'Y METHOD FOR ELECTROSTATICALLY HOLDINGPARTICLES ALONG A MOLD SURFACE BACKGROUND OF THE INVENTION 1. Field ofInvention This invention relates to a method of sinter molding, and moreparticularly the employment therein of electrostatic means to aidretention of comminuted material on the mold surface.

2. The Prior Art Electrostatic deposition methods and means are wellknown for use in coating surfaces of metal, paper and other materials.Usually such coatings are relatively thin, on the order of a few mils.Electrostatic deposition has been found satisfactory for suchapplications.

It has been proposed to adapt electrostatic deposition of comminutedmaterial for sinter molding to form such articles as plasticreceptacles. It has also been proposed in certain such applications touse roll deposition apparatus, which is found advantageous fordepositing large quantities of material in a short time. However, priorproposed methods and apparatus for electrostatically charging powderstreaming from a roll deposition unit have proved unsatisfactory,particularly with high-capacity units. Furthermore, a strongelectrostatic field in the vicinity of the roll causes material to stickor clump in the unit. Various schemes for grounding, shielding orcountercharging of the roll, case and other parts have proved to becomplex, costly and generally unsatisfactory.

In the case of a high deposition rate, as required for good productioneconomy, fusion alone often will not effect sufiiciently completeparticle adherence or coherence on impingement to preclude dislodgementof some particles from their points of deposit on the mold, or form.Such dislodgement, by falloff, windage, vibration or other operatingconditions, results in erratic distribution and assimilation of thematerial, precluding consistent uniform molding to the desiredconfiguration. Irregular distribution of the material exhibits itself inrough surfaces, thin spots and porosity of the article.

SUMMARY OF THE INVENTION It is an object of this invention to provide amethod for fostering adherence and uniform build-up of plastic powderprogressively deposited on a substrate, such as a mold surface or thelike.

It is a further object of this invention to provide a method foreffecting retention of comminuted material upon a form or mold surfaceby electrostatically charging the material immediately subsequent todeposition on the substrate, outside the particle stream and depositionzone.

It is a still further object of this invention to provide in associationwith a method for sinter molding hollow articles in a cylindrical mold,a method for applying an electrostatic charge to freshly depositedpowder so as to attract unassimilated particles to the mold and thusretain such particles at their respective points of deposition,notwithstanding effects of gravity, windage and vibration, at leastuntil such particles fully assimilate with the other material along themold surface.

The foregoing and other objects and advantages are achieved by a novelmethod wherein particles carried on a form or mold are passed through anelectrostatic field immediately subsequent to deposition of theparticles on the form. The field is maintained at a potential differentfrom that of the form or mold.

The invention is exemplified in a preferred embodiment as an arrangementfor sinter molding a plastic drum liner, wherein a heated, opencylindrical mold is rotated with its axis horizontal and a stream ofpowder is directed downwardly on the mold surface in a strip zone alongthe bottom of the mold. A corona bar is positioned outside the case ofthe deposition unit, parallel to and coextensive with the effectivelength of the deposition roll, and thus coextensive and parallel to thestrip deposition zone. The corona bar is positioned so as to oppose thewall of the mold most closely just beyond the deposition zone. The moldis grounded. An electrostatic charge on the bar sets up a field havingan ionizing envelope of highest intensity generally parallel to andoutside the deposition stream, between the bar and the mold. The freshlydeposited powder carried on the mold surface passes through thehighintensity ionin'ng envelope, whereby unassimilated particles arecharged and are attracted to the mold wall with sufficient force tocounteract the forces of gravity, windage and vibration tending todislodge such particles. Therefore, such relatively free particles donot fall back along the mold wall toward the deposition zone, where theywould otherwise collect and tumble randomly until fused.

The charging potential difference is such that the uncoalesced particlesremain charged and adherent where deposited, until they are permanentlyassimilated by fusion and coalescence. The charges then dissipatethrough the grounded mold so that substantially the full potentialdifference is effective on the next pass to maintain the field betweenthe corona bar and the mold with material fused along the mold surface.In this manner substantially all particles are retained at theirrespective points of deposit along the mold wall, according to thepattern predetermined by the deposition unit, which pattern isrelatively unaffected by electrostatic disturbance. A homogeneousarticle of predetermined thickness or wall section is thus obtained withan arrangement which is readily adaptable to automatic production.

The foregoing features, objects and advantages of the invention will bemore fully understood from the ensuing detailed description read withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a schematic perspective viewof novel apparatus in accordance with this, invention, for practicingthe novel method in association with formation of a receptacle in anopen, hollow mold;

FIG. 2 is a section on line 2-2 of FIG. 1;

FIG. 3 is a fragmentary perspective view along one side of thedeposition case, showing one of the adjustable brackets for supportingthe corona bar; and

FIG. 4 is a schematic, partial cross-section of the apparatus on anexaggerated scale, showing the efiect of electrostatic charges on theprogress of assimilating comminuted plastic material in the mold shownin FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENT The invention is herein described inassociation with sinter molding to form the cylindrical wall of a drumliner from comminuted electrically non-conductive plastic material, suchas polyethylene. A representative liner is about 2 k feet in diameter X3 7% feet long, with a wall thickness of about 1/32nd to l/l6th ofaninch.

As seen in FIG. 1, cylindrical mold 10 is positioned with its axishorizontal, for rotation about its axis in a counterclockwise directionas viewed from the forward end of the mold. A roll deposition unit 11 ispositioned axially of the mold. The deposition unit consists of a case12, housing an auger 13 for feeding powder lengthwise of the case, anddeposition roll 14 parallel to and immediately beneath the auger, forstreaming powder downwardly onto the wall of mold 10 throughoutsubstantially the full mold length along line 15-15 at the bottom of therotational path.

The mold is heated in any suitable manner, by burners, or by jacketheating with steam or hot water, for the purpose of rapidly fusingpowder deposited in the mold. Particular heating means is not part ofthis invention. The mold temperature is maintained at a value sufficientto efi'ect fusion of powder particles on their near approach to the moldsurface, so that the material incipiently fuses and assimilatessubstantially as rapidly as deposited. Because of aeration andunavoidable variation in the distribution of particles within the streamfalling upon the mold, there will usually be some variation in thefusion rate among the particles, particularly after some thickness hasbeen built up in the course of repeated mold rotation and associatedprogressive deposition. Accordingly,

in the path just beyond the deposition zone, where the mold wallprovides substantial upward support.

With usually suitable plastic materials, it is not feasible to solve theproblem of particle fallout by increasing the mold temperature, as anunduly high temperature may result in overly rapid degradation, bumingor other deleterious effect on the material, and consequent defects inthe article. An acceptable fusion rate can be established which issufiicient to preclude significant particle dislodgement for a shortdistance from a line of deposit along the bottom of the mold path, butfallout usually becomes excessive as the freshly deposited materialapproaches and crosses the horizontal plane, where there is no directsupport. Dislodgement or fallout is usually irregular, resulting in anirregularity of the material distribution along the mold wall, withcorresponding pebbling, thick and thin spots, pin-holing, voids andsimilar defects in the resultant article.

As previously observed, the expedient of charging the stream does notprove efficacious in a sinter molding operation such as that hereininvolved. Good production economy requires deposition at anexceptionally high rate, with a correspondingly dense, large stream ofpowder at relatively low particle velocity. Charging the particles inthe deposition stream by means of a practicable electrostatic chargingapparatus does not produce a consistent charge throughout the stream.Particle flow is correspondingly erratic, especially at the lowvelocities involved in roll deposition. In the case of a moldingoperation as illustrated in FIG. 1, a fairly high effective potentialdifference is required to insure retention of particles travellingupward. A corresponding charge imposed on particles in the depositionstream not only gives rise to the crratic flow conditions described, butis wasteful of energy, since much or most of the material willsatisfactorily adhere along the mold without the electrostatic assist.

FIGS. 1 and 2 show the arrangement according to this invention, wherebyto charge material subsequent to deposition, with an electrostatic fieldin an ionizing envelope beyond the deposition zone. Corona bar 16 isprovided for this purpose, mold being grounded as shown. Powder fallsfrom roll 14 at a predetermined rate, according to the speed of roll 14and the setting of blades 18 and 19, forming a stream 23, which impingeson mold 10 along the line 15l5 (FIG. 1), within a zone of arcuate extentZ.

Bar 16 is supported on a pair of brackets, 21, 22 which in turn areaffixed to the deposition roll case 12. A length of band saw blade is asuitable bar, with the edge most closely opposing the wall of mold 10immediately below bar 16, providing a directional emission of peakintensity along a line of the mold outside the deposition zone andparallel thereto.

Corona bar 16 is so positioned that the closest communication to groundis through the grounded mold 10 along its element most closely oppositethe bar 16. An electrostatic charge on bar 16 establishes a field havinga peak intensity envelope between bar 16 and mold 10, through whichenvelope material is carried on mold 10 in its path away from depositionzone 2. Unassimilated particles accept charges from the field, settingup attractive forces between such particles and the mold, to resistdislodgement by gravitational, windage and other forces acting on theparticles. In the case of the operating conditions here described acharge of 20 kv on bar 16 and a gap X of about I-% inches establish asuitable field. A gap at least about twice the distance X is maintainedbetween bar 16 and powder 17 in case 12, assuring against straydischarge tending to cause powder to stick on roll 14. Bar 16 is alsosufiiciently remote from any part of mold 10 on a line across stream 23to insure that the field is of substantial intensity primarily outsidethe stream, minimizing electrostatic disturbance of the stream and thepattern of distribution on mold 10 along arc Z. The electrostaticdischarge thus focuses along a line of peak intensity through point P,parallel to the axis of mold 10, but remote from the deposition zonealong arc Z.

In order to assure an optimum combination of electrostatic fieldstrength and focus of peak intensity for given operating conditions, theposition of bar 16 is adjustable on two axes relative to unit 11.Operating variables contemplated include the distance of roll 14 fromthe mold 10, the rate of deposition, the rotative speed of mold l0, anduse of the deposition unit with other mold sizes. Correspondingadjustments of bar 16 are provided by adjustability of the brackets 21,22 laterally and vertically. t

The construction of bracket 22 is shown in FIG. 3, bracket 21 beingsimilarly constructed. Bracket 22 comprises two angles 24 and 25. Theupright leg 26 is fastened to case 12 by means of cap screw 27,extending through slot 28. The horizontal leg of angle 24 is clamped toangle 25 by cap screw 30 passing through slot 31. Thus, slot 28 providesfor vertical adjustment of bracket 22 and bar 26, while slot 31 provideslateral adjustment. Brackets 21 and 22 are rubber-coated or insulated inany suitable manner, to obviate shorting or arcing.

Operation can best be understood by reference to FIG. 4, representing astage at which a substantial deposit has been built up along moldsurface 32 in the form of a blanket 33 of sintered material, brought toand maintained in sintering state by heating mold 10, as indicatedschematically by the burner 34. Mold 10 rotates counterclockwise, asviewed in FIG. 4. While mold 10 rotates, with blanket 33 maintained atsintering temperature, the stream of material 23 is projected downwardfrom roll 14 onto the blanket 33. Most of the particles deposited fromstream 23 assimilate virtually instantaneously with the previousdeposit, at least to such degree as to become substantially inseparable.However, due to unavoidable variations in distribution of particles instream 23 and associated non-uniformity of particle heating and fusionrate, random particles do not instantaneously assimilate with blanket33, but only lightly adhere on the surface, as represented by particle35, here enlarged for illustrative purposes. Under conditions of highspeed production and correspondingly high deposition rate, theproportion of such unassimilated particles 35 usually is fairlysubstantial. Particles thus lightly adhered are generally so delayed infusion and assimilation that they tend to separate from blanket 33 androll back along mold 10, due to the forces of gravity, windage andvibration, as the point of deposit on surface 32 moves along the upwardarc of rotation, absent an auxiliary attracting force. Such dislodgementor fallout is random, resulting in irregular formation of the article.This objectionable condition tends to become even more aggravated as theoperation proceeds, with free particles in increasing number tumbling atthe bottom of the mold and disrupting assimilation of fresh materialfrom stream 23.

Assimilation of such lightly adhered material at the place of depositalong surface 32 is fostered by charging the particles while stillsubstantially supported by the mold, thereby efiecting a retentiveattraction between unassimilated particles and mold 10. Generator 41impresses an electrostatic charge on corona bar 16, establishing thefield having peak intensity in ionizing envelope 40, here shown aspositive in sign, with the grounded mold 10 negative. As particle 36passes through ionizing envelope 40, the particle is charged andstrongly attracted toward grounded surface 32, thus adhering particle 36in its position of rest on blanket 33 causing such particle to adhere tothe deposit blanket 33 by virtue of attractive force between theparticle and the mold 10.

The nature of the electrostatic effects is illustrated by the schematicrepresentation of a series of particles, such as particle 35, inprogress around the path of rotation or mold 10. Particle 36 is passingthrough ionizing envelope 40 and is there charged. Charged particle 37remains adhered to the blanket 33, as assimilation sets in. Particle 38is partially assimilated and particle 39 has been fully assimilated. Asassimilation proceeds, with accompanying electrical conductivity betweenthe blanket 33 and mold 10, particle charges are dissipated at a rategenerally corresponding to the rate of particle assimilation.Observations indicate that in the example here described the charges aresubstantially dissipated within one revolution, particles becoming fullyfusion-adherent within a fraction of revolution. With the blanket 33 atall times grounded through the mold 10, requisite electrostatic fieldstrength for efiectively charging free, unassimilated particles asdescribed can be maintained with a relatively low demand on thegenerator 41.

That portion of the material which is fully assimilated is substantiallyat ground potential on approach to the electrostatic field. By virtue ofgood electrical communication between the mass of the deposit and themold wall, the coalesced material is substantially fully de-ionized whenpassing through the deposition zone and approaching the field. Thiscondition minimizes any repellent effect between the general mass of theplastic material and unassimilated or imperfectly coalesced particlesalong the surface. Because of substantial electrical discontinuity, thefree particles accept the electrostatic charge and are attracted to themold wall and to the other material with sufficient force to precludedislodgement, until fully assimilated as described.

The practice of the invention is not limited to the preferred embodimentabove described. The invention is also useful in association withdeposition on mandrels, male molds, webs, and other forms. Those skilledin the art will perceive other uses, variations, and modificationswithin the spirit and scope of the invention as defined in the appendedclaims.

What is claimed is:

1. ln a method of molding an article from comminuted particles of anelectrically non-conductive material upon a mold surface to which saidparticles are fixed in place during said molding, including the steps ofdepositing a layer of said comminuted particles along successiveportions of said mold surface, and fusing said particles to each otherfor forming a blan- A Inn ket of said material upon said mold surface;the improvement comprising:

imposing an electrostatic charge to said layer of particles onlysubsequent to performing said step of depositing said particles forcausing said particles to be electrostatically fixed in place relativeto said mold surface until said pardcles have assimilated in saidblanket.

2. A method as defined in claim 1 wherein said particles are assimilatedinto a blanket by the step of heating said mold to a temperature atleast as high as the melting temperature of said particles prior to thestep of depositing said particles.

3. A method as defined in claim 1 wherein the step of depositing saidparticles is accomplished with the aid of gravity, and includes the stepof rotating said mold about said particles which are being deposited forcarrying deposited particles past a source of electrostatic chargeslocated remote from the point at which said particles are beingdeposited.

4. In a method of forming a hollow cylindrical plastic member, includingproviding a mold having a cylindrical cavity, heating said mold,rotating said mold about the axis of said cavity, and directing a streamof comminuted plastic material against the wall of said cavity in adeposition zone so as to effect assimilation of said material along saidwall, the improvement comprising positioning a corona bar in said cavitywith said bar extending lengthwise of said wall coextensively with saidzone and said bar closest to said wall along a line thereof outside saidzone, and creating an electrostatic discharge between said bar and saidwall so as to concentrate charging effect of said discharge on saidmaterial largely along said line, thereby charging particles of saidmaterial carried on said wall upon crossing said line and effectingattraction of said particles to said wall free of significantelectrostatic effect in said zone.

5. The method as defined in claim 4, said bar being elongated parallelto said axis and outside said stream and positioned closest to said wallin a plane common to a line of said wall immediately beyond said zone inthe direction of mold rotation, including grounding said mold, andimpressing an electrostatic charge on said bar.

1. In a method of moldIng an article from comminuted particles of anelectrically non-conductive material upon a mold surface to which saidparticles are fixed in place during said molding, including the steps ofdepositing a layer of said comminuted particles along successiveportions of said mold surface, and fusing said particles to each otherfor forming a blanket of said material upon said mold surface; theimprovement comprising: imposing an electrostatic charge to said layerof particles only subsequent to performing said step of depositing saidparticles for causing said particles to be electrostatically fixed inplace relative to said mold surface until said particles haveassimilated in said blanket.
 2. A method as defined in claim 1 whereinsaid particles are assimilated into a blanket by the step of heatingsaid mold to a temperature at least as high as the melting temperatureof said particles prior to the step of depositing said particles.
 3. Amethod as defined in claim 1 wherein the step of depositing saidparticles is accomplished with the aid of gravity, and includes the stepof rotating said mold about said particles which are being deposited forcarrying deposited particles past a source of electrostatic chargeslocated remote from the point at which said particles are beingdeposited.
 4. In a method of forming a hollow cylindrical plasticmember, including providing a mold having a cylindrical cavity, heatingsaid mold, rotating said mold about the axis of said cavity, anddirecting a stream of comminuted plastic material against the wall ofsaid cavity in a deposition zone so as to effect assimilation of saidmaterial along said wall, the improvement comprising positioning acorona bar in said cavity with said bar extending lengthwise of saidwall coextensively with said zone and said bar closest to said wallalong a line thereof outside said zone, and creating an electrostaticdischarge between said bar and said wall so as to concentrate chargingeffect of said discharge on said material largely along said line,thereby charging particles of said material carried on said wall uponcrossing said line and effecting attraction of said particles to saidwall free of significant electrostatic effect in said zone.
 5. Themethod as defined in claim 4, said bar being elongated parallel to saidaxis and outside said stream and positioned closest to said wall in aplane common to a line of said wall immediately beyond said zone in thedirection of mold rotation, including grounding said mold, andimpressing an electrostatic charge on said bar.